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The most significant event in the 2012 presidential election remains the Romney miracle bump after the first debate. If Mr. Romney wins the election, analysts and scholars will spend years picking apart the Denver debate the way they have the 1960 Kennedy-Nixon debate.

Richard Nixon didn't lose that election because of his five o'clock shadow, and Barack Obama isn't going to win or lose his presidency because he lacks intensity. What we learned on Long Island is that Mr. Obama lacks something more damaging to an incumbent—a sense of presidential responsibility.

One of the most familiar Obama positions—repeated at every campaign stop—is that he "inherited" a bad economy from George W. Bush. Set aside that whatever the cause, everyone concedes he took over a tough situation. More to the point is Mr. Obama's compulsive insistence that anything awry in the economy during his first term is "not my fault."

The Bush-did-it narrative was a banality by the time of the debates. Then came Benghazi. Within days, the political question at the center of the incident was: What did the White House know and when did it know it? No matter one's politics, it became impossible not to see that the White House was intent on putting "distance" between the president and responsibility for the security breaches.

ENLARGE

Barack Obama at the presidential debate in Hempstead, N.Y.
Bloomberg

Vice President Biden in his debate with Paul Ryan explicitly transferred early responsibility to some offshore cloud called "the intelligence community." Then this week, Secretary of State Clinton accepted formal responsibility. By now, this had the look of Hillary taking the fall for the president's candidacy.

So came the moment late in the Hofstra debate when moderator Candy Crowley looked at Mr. Obama and asked: "Does the buck stop with your secretary of state as far as what went on here?"

Staring back, the president clutched for a second. He looked like a fourth-grader being confronted in front of the whole class by Miss Crowley of all our childhood nightmares. That moment revealed the problem: At the core of Barack Obama's persona and his presidency is a constant instinct to deniability.

It's not my fault. He comes across as one of those smart kids who always had some elaborate excuse to disperse responsibility for anything bad in his vicinity. And so it was in his answer to Miss Crowley: "Secretary Clinton has done an extraordinary job. But she works for me. I'm the president. And I'm always responsible. And that's why nobody is more interested in . . ." By the end, he said it was Mitt Romney's fault for bringing it up! In contrast, the bin Laden takedown was accompanied by a Lady Gaga-like White House P.R. blitz in the media.

In hindsight, an irony of the 2012 campaign will be that Barack Obama and Mitt Romney traded places on stepping up to the plate. A main criticism from the right of Mr. Romney had been that he was playing it too safe, saying next-to-nothing about much of anything, such as his tax returns, for fear the Obama camp or the press would criticize him. No exposure, no responsibility.

That flipped in Debate No. 1. Mitt Romney exploded out of his turtle shell—and Barack Obama disappeared into his. Mr. Obama's supporters want us to think that defensive performance was a one-off. It's not. To be sure, Barack Obama is no shrinking violet. He loves the bright lights of the presidency. What he can't handle are the embarrassments that come with it.

The president's current difficulties aren't merely about instinctively stepping away from bad news. Another, unavoidable burden of the presidential office—this being a democracy—is the necessity to engage political adversaries. But it was noted before the first, disastrous debate that Mr. Obama's opinion of Mr. Romney was so low, for some unexplained reason, that he didn't think he was fit to be president. So why stoop to debate him?

This was of a piece with his famous speech at George Washington University in April 2011, when with the GOP congressional leadership seated before him to hear the administration's counterproposals on reducing the deficit, Mr. Obama instead mocked them as "nothing serious." Whatever else, that's not presidential.

That former Obama voter at Hofstra who asked the president to respond to the state of "everyday living" in the U.S. deserved better than a reflexive deflection into ending the Iraq war and how "we saved an auto industry."

For much of the American electorate, this began as an ideal presidency. But there is an institutional flaw at the center of Mr. Obama's understanding of the presidency. He accepts the best of it but not responsibility for the inevitable worst of it. It is making his incumbency smaller than he thinks it is. His misfortune is that in the election's last lap, the public has begun to notice.

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